This invention relates, in general, to radar systems and, more specifically, to radar systems using continuous wave transmissions and to processing techniques for increasing the dynamic range of the radar receiver.
Continuous wave (CW) radar systems operate with a constant amplitude transmitted signal usually phase or frequency modulated by a predetermined waveform to aid in measuring the time delay of the echo signals. Since the signal is always being transmitted, as opposed to pulse radar systems, sufficient isolation must exist between the transmitted signal and the received echo signals. Using separate transmitting and receiving antennas is helpful, but additional cancelling of the transmitted signal is needed. Filters and phase reversal cancellation techniques are commonly used for this purpose, but still a significant amount of energy leaks from the transmitter to the receiver circuits. As a consequence thereof, receiver circuits have been required to exhibit large dynamic ranges in order to handle both the relatively weak echo signals and the relatively strong leakage and short range clutter signals.
Traditional filtering and phase reversal cancellation techniques for reducing the leakage and clutter signals often require apparatus having precisely maintained electrical and/or mechanical parameters. The problem is complicated by the desire to increase the range of CW radar systems, thus requiring more transmitted power and either more filtering or cancellation, which is already at the practical limit. The problem is mitigated by using radar receivers capable of wide dynamic range which adequately process the weak echo signals along with the strong leakage and clutter signals. Therefore, it is desirable, and it is an object of this invention, to provide a CW radar system which has wide dynamic receiver response.
Several issued patents describe radar systems which use some of the techniques taught by the invention disclosed herein. U.S. Pat. No. 3,344,426, issued Sept. 26, 1967, describes a radar system which uses chirp or variable frequency modulation with multiple receiving channels for the purpose of changing the illumination energy at each elevation angle of a stacked beam radar system. The present invention varies the modulation frequency of a continuous wave radar transmitter substantially in a linear fashion and has other novel elements which distinguish over this patent.
U.S. Pat. No. 3,212,087, issued on Oct. 12, 1965, describes a pulsed radar system with range resolution enhancement provided by multiple receivers sensitive only to a portion of the pulse signal. The present invention achieves greater dynamic range in a digital radar receiver by blocking out receiving channels in synchronism with the original modulation, and by other features to be described in detail later herein.
U.S. Pat. No. 3,188,637, issued on June 8, 1965, describes a radar system which utilizes chirp modulation, narrow band filtering sections, and gating circuits. The receiving mixers are injected with a chirp or linearly variable sawtooth signal which, when mixed with the echo signals, produces a fixed, not varying, intermediate frequency which is applied to the filters. The present invention uses the linear swing of the radar signal to successively apply the signal to different receiving channels which are selective to different frequencies. This could not be done according to the teachings of the reference patent since the IF signal frequency is constant.